Technical Data

Capacitor Theory

Capacitor Theory

A capacitor basically consists of two plates with an insulator in between,
although in practice the ‘plates’ are normally rolled up in a can to save space.
It can be used in a circuit to store charge for small periods of time.

Charge stored in a capacitor:

Charge Q = CV where C is the capacitance in Farads

charge Q is measured in coulombs (C)

Energy stored in a capacitor:

Energy stored, W = ½ QV = ½ CV² joules

Capacitance:

If the dielectric (the material between the plates) is a vacuum, Capacitance
C = e₀ (A / l) where A is the area of the
capacitor plates, and l is the distance between them.

e₀ is the permittivity of free space
(8.85X10-12)

If the dielectric is another material, capacitance is given by:

C = e_re₀
(A / l) where e_r is the relative
permittivity, which varies between materials.

Capacitors in Series:

Putting capacitors in series reduces the overall capacitance:

(1/C) = (1/C₁) + (1/C₂) + (1/C₃) …..

Capacitors in parallel:

Putting capacitors in parallel increases the total capacitance:

C = C1 + C2 + C3 …..

Note that the series and parallel capacitance formulae are the opposite of
those for resistance.

Capacitor Markings

Capacitor Markings

Capacitors are often marked with codes to show the value, tolerance and
material. This is particularly true for small types such as ceramic disc or
polystyrene where there is little space for full markings.

Value Codes:

The capacitance value is often marked using a 3 digit code. This works in the
same way as resistor coding but using numbers instead of colours. The first 2
numbers give the value and the last number is the multiplier.

These give the
value in Picofarads (pF), e.g. code 103 = 1 0 000pF (=0.01uF – see Capacitance
Conversion Table). Alternatively the value may be marked directly, for
example 2n2 is 2.2 Nanofarads (nF).

Tolerance Code:

A single letter is often used to indicate the tolerance of the component.
These can be translated using the following table:

Material Code:

The dielectric material is often marked in abbreviated form. The table below
shows the meaning of these abbreviations.

Capacitance Conversion Table

Capacitance Conversion Table

Different types of capacitors often have their values marked in different orders of magnitude. It is sometimes necessary to convert between them:

AC Power

Apparent Power, S – appears on the hypotenuse of the power triangle.

Apparent power is given by:

Apparent power = V I

= Voltage * Current

Units are volt -amps (VA)

True power is given by:

I²R = S cos f = VI cos f

This is the power in the resistive part of the circuit

Units are Watts (W)

Given by:

I²X_L = S sin f = VI sin f

and: I²X_C = S sin f = VI sin f

where X_L and X_C are inductive reactance and capacitive reactance respectively, and f is the phase angle.

Units are volt-amps reactive (VAr)

Power Factor = True Power / Apparent Power = P / S = cos f

True Power  = Apparent Power * Power Factor

= VI cos f